1. Field of the Invention
The present invention is generally in the field of semiconductor fabrication. More specifically, the invention is in the field of fabrication of bulk acoustic wave structures in semiconductor dies.
2. Background Art
Bulk acoustic wave structures, which can be used in frequency control or filtering applications, can include a piezoelectric layer and an acoustic mirror structure. The acoustic mirror structure, which can include a number of alternating low and high acoustic impedance layers, can be used to trap acoustic energy in the piezoelectric layer. In the acoustic mirror structure, the level of acoustic reflectivity at the interface between a low and a high acoustic impedance layer determines the number of alternating low and high acoustic impedance layers the acoustic mirror structure requires to achieve a desired level of acoustic reflectivity. By way of background, the level of acoustic reflectivity at an interface between two layers is determined by the difference in each layer's acoustic impedance. Also, a denser layer generally has higher acoustic impedance than a less dense layer. Thus, the level of reflectivity between low and high acoustic impedance layers in the acoustic mirror structure can be increased by decreasing the density of the low acoustic impedance layers and increasing the density of the high acoustic impedance layers.
In a conventional acoustic mirror structure in a bulk acoustic wave structure, silicon dioxide, which has a very low density, can be used in low acoustic impedance layers and metal, which is a readily available high density material, can be used in high acoustic impedance layers. For example, tungsten, which is easy to deposit, relatively inexpensive, and has a high density, is a typically used metal in high acoustic impedance layers in conventional acoustic mirror structures. However, high density metal such as tungsten also have relatively low resistivity, which can impose or worsen undesired RF parasitic paths detrimental to device function including stray capacitances and image currents in the high acoustic impedance layers of the acoustic mirror structure due to its close proximity to the device's signal path. These induced parasitics can cause an undesirable increase in electrical loss in the bulk acoustic wave structure. Additional loss in bulk acoustic wave structures can be caused by scattering of acoustic waves at non-smooth interfaces between the layers of the structure, and acoustic viscosity seen by the waves traveling through the layers. Each of these sources of loss can also be reduced by proper choice of composition of high acoustic impedance layers.
Although insulators such as aluminum nitride or tantalum oxide can be used in high acoustic impedance layers in conventional acoustic wave structures, these insulators generally have a much lower density than high density metals, such as tungsten. As a result, conventional acoustic wave structures that use insulators in high acoustic impedance layers typically require more additional low and high acoustic impedance layers to achieve a desired level of acoustic reflectivity, which undesirably increases manufacturing cost. It is also the case that non-optimal choices for high and low impedance layer materials will reduce the observed electromechanical coupling of the bulk acoustic wave structures and as such reduce the bandwidth of filters in which they are used.
Thus, there is a need in the art for an acoustic mirror structure for bulk acoustic wave structures, where the acoustic mirror structure includes high acoustic impedance layers comprising a high density material that does not cause increased electrical loss in the bulk acoustic wave structures.